In a first known method of reducing the vibration of a structure, and thus the noise transmitted by said structure, an additional heavy plate, e.g. based on lead, is placed against the structure so as to improve the vibratory and/or sound insulation of said structure by the mass effect.
That first method is very effective but it presents the drawback of being heavy. That drawback is a major drawback in the field of aviation, where weight always needs to be optimized.
In a second method, the vibration, and thus the noise generated by a structure, is reduced by providing damping that serves to dissipate the vibratory energy, converting it into heat.
A first system implementing that second method consists in a monolithic plate of elastomer arranged on the structure. The damping power of the elastomer then reduces the dynamic deformation of the structure, whatever the mode of the mechanical stress to which it is subjected, and in particular it dissipates energy in the form of heat.
Compared with the first method, the saving in terms of weight is considerable, since elastomer is not very heavy. Nevertheless, the efficiency of that first system is limited since oscillation of the structure gives rise only to small movements in the elastomer. Thus, energy dissipation in the elastomer plate is weak.
To improve that first system, it is common practice to use a second system. The second system comprises a visco-constrained elastomer, i.e. an elastomer plate having its top portion previously bonded to a metal support. The bottom portion of the elastomer plate, opposite from its top portion, is then fastened to a structure.
During dynamic deformation, e.g. bending of the structure as generated by vibration, the top face is held by the metal support. That additional condition at the limits induces a state of internal stresses in the elastomer that is greater than the above-described system. Consequently, the amount of vibratory energy that is dissipated in the elastomer plate is increased.
The second system is certainly more efficient than the first system. Nevertheless, for applications that require a large amount of vibratory energy to be dissipated, it is found to be still insufficient. Furthermore, it can turn out to be heavy.
Document FR 2 870 308 discloses a third damper system.
The absorbent coating of FR 2 870 308 is fastened to a structure and is provided with an absorbent layer of an elastic material that includes a dissipater mesh. The mesh is arranged in the elastic material and is constituted by a plurality of nodes and of dissipater elements.
In such a device, the vibratory energy that is absorbed is dissipated mainly in the form of heat by deforming material.
The energy dissipation in the absorbent layer is considerable since the elastic material of the absorbent layer is deformed both by a lever arm effect caused by the nodes that hold the dissipater elements apart from the structure, and by the dissipater elements that cause the stresses that are transmitted by the nodes to be spread throughout all of the elastic material, while amplifying them by a geometrical effect.
That system is very effective, but the absorbent layer represents a weight that is not negligible.
Finally, document FR 2 909 740 presents a fourth damper system. Document FR 2 909 740 provides an absorbent coating having high damping power, the coating comprising a dissipater mesh made up of a plurality of dissipater elements and of nodes, the bottom end of each node being fastened to the structure. That coating is remarkable in that the bottom end of each node projects relative to the dissipater elements so as to create an empty space between the dissipater elements and the structure, so that the absorbent coating is provided with main absorbent elements that are arranged in said empty space, these main absorbent elements being secured firstly to the bottom face of the dissipater elements and secondly to the structure.
The performance of the absorbent coating is very advantageous. Nevertheless, it is found that it is difficult to mass produce, thereby sometimes making it awkward to use.